[twothamendment]

I'm in Northwest Montana. My ground source heat pump doesn't struggle until the highs outside are -20F (actual, not wind-chill). I have the backup heat strip, but the breaker is off. I don't know when it would turn on, I just wanted to know it wouldn't without me knowing it.

[glxxyz]

I'm in Canada at a similar latitude with ground source, resistance heating normally kicks in at about -25C (-13F) or so, just a few hours on the coldest nights, doesn't cost much. I could probably leave the breaker off too, I wouldn't mind it a degree or two colder.

[JeremyPOsborne]

Ground source is often the only choice here. While air source can technically work well down to these temperatures, much of the available equipment doesn't suit some homes.

[maxerickson]

Ground source is largely going to maintain capacity independent of the outside temps, so the resistive would turn on when the heat pump isn't keeping up with the heat loss.